Present day space solar cells require that accurately calibrated precision equipment be used for tests and measurements of the solar cells prior to operational use. During testing of the solar cells, an important testing device is the light source which is used to simulate solar radiation upon solar cells during testing. Advanced solar cells are extremely sensitive to the spectral content of the incident light. Therefore, it is necessary to test the solar cells under a light with spectral characteristics similar to those of the sun in space defined as the well known Air Mass Zero (AM0) solar spectra. The use of a well matched AM0 spectra is particularly important for testing multi-junction solar cells which have irregular performance output over the solar spectral bandwidth.
Energy integration of the solar spectra over the complete spectral bandwidth of 0 to 1.0 meters in wavelength, yields a solar energy constant of 135.3 mW/cm.sup.2. Energy integration of the solar AM0 spectra over an effective operational bandwidth range of 250-2400 nm wavelengths, yields an energy constant of 129.27 mW/cm.sup.2, which is 95.5% of the total integral solar energy. A scaled irradiation spectrum for a 5800K blackbody is approximately equal to the integral solar energy over the 250-2400 nm wavelength range. That is, the solar spectral curve shape at one astronomical unit, the earth to sun distance, is approximated by an intensity scaled 5800K blackbody. A 3200K blackbody irradiation spectrum, the spectrum of a tungsten source, with nearly equally scaled integral energy over the 250-2400 nm wavelength range, does not match the solar spectral curve. As the temperature of a blackbody decreases, the peak energy decreases in intensity and shifts towards longer wavelengths. Thus, a cooler blackbody spectrum has a greater portion of energy in the infrared (IR). This spectral mismatch is well known as early solar simulators used only tungsten sources. Variations between different blackbody spectra are also important when considering the use of a tungsten source to match the IR portion of AM0. The tungsten source, while providing equally scaled integral energy matching over the entire 250-2400 nm wavelength range, disadvantageously has lower energy levels in the UV and visual 250-900 nm range and higher energy levels in the near IR and IR 900-2400 nm range. Hence, the tungsten source is not a good single source solar simulator.
Conventional solar simulators have been based on xenon arc lamps having high energy levels in the UV, visual and near IR ranges and lower energy levels in the IR ranges as compared to the tungsten source. Testing of solar cells has been done using xenon arc lamps. Xenon arc lamps are known to have a nearly constant intensity throughout the visible portion of the spectrum. The xenon arc lamps are also known to disadvantageously have several high intensity spikes, particularly between 700 and 1100 nm wavelengths, for example, the xenon arc lamp used in a prior art XT-10 xenon solar simulator. The high pressure xenon arc lamps have several high intensity spikes of energy which can greatly affect solar cell measurements. The presence of high intensity spikes in a solar simulator spectrum can cause significant variations between terrestrial test solar cell measurements and actual performance in space. The high intensity spikes can cause erroneous performance measurements of single-junction and multi-junction solar cells caused by varying temperature coefficients due to the semiconductor band gap and absorption variations with temperature. There has been a great deal of effort aimed at filtering the xenon arc lamp spectra to better match the AM0 spectrum by removing the high intensity spikes, but their complete elimination has been largely unsuccessful. Despite the great deal of effort that has been directed toward reducing the intensity of the xenon spikes through optical filtering, spike elimination has not been successful resulting in low spectral fidelity.
Several xenon arc lamp solar simulators have been developed but have either high intensity spikes or mismatch to the AM0 spectrum, or both. The spectral intensity of a Large Area Pulsed Solar Simulator (LAPSS) as a function of wavelength has poor solar spectra matching. The LAPSS has fewer high intensity spikes than the XT-10. However, the general shape of the LAPSS spectral curve does accurately match the AM0 spectrum. An Oriel xenon arc lamp solar simulator with an AM0 filter better matches the solar spectrum. However, the Oriel solar spectra has several high intensity spikes. A Spectrolab X-25 solar simulator has only one high intensity spike over the 300-1100 nm wavelength range. The spectrum of the X-25 over a wider wavelength range has several high intensity spikes beyond 1100 nm that can affect solar cell measurements. The single xenon source X-25 solar simulator has improved performance over the 300-1100 nm range, but uses 19 different filters aimed at both reducing the high intensity spikes and providing a uniform large area illumination spot size. While the X-25 has several high intensity spikes, the general shape of the curve nearly matches that of AM0. The X-25 has been widely used throughout the space solar cell testing community.
Prior art spectral measurements have been made using silicon photodiode detectors that are only sensitive to light in the 300-1100 nm range. In the past, this was an adequate measurement of a solar simulator since most solar cells being tested were also made from silicon. Therefore, a solar simulator only had to match a limited portion of the AM0 solar spectrum. Some advanced solar cells use lower band gap materials which are sensitive to a larger portion of the solar spectrum and therefore increase the solar spectra matching requirements of a more accurate solar simulator. In particular, triple-junction solar cell designs have included active Ge junctions which absorb light having wavelengths up to 1880 nm.
A prior art Hoffman dual source solar simulator uses a xenon arc lamp to match the ultraviolet (UV) and visual portions of the AM0 spectrum and a tungsten lamp to match the near IR and IR portions. This prior art dual source simulator was designed to match the solar spectral irradiance as earlier defined but which differs slightly from the presently accepted AM0 measurements. This prior art Xe-W (xenon and tungsten) dual source solar simulator was developed to provide a better match to the AM0 spectrum by taking advantage of the UV and visual spectra of the xenon arc lamp and the near IR and IR spectra of the tungsten lamp. However, this prior art Xe-W dual source solar simulator does not completely remove the high intensity spikes of the xenon arc lamp and provides poor spectral matching over the entire 250-2400 nm wavelength range. Problems with this prior art Xe-W dual source solar simulator are better understood by reference to FIG. 1. It is important to compare the prior art Xe-W dual source solar simulator to the AM0 spectra. FIG. 1 depicts the AM0 solar spectra compared to the spectrum of the prior art Xe-W dual source solar simulator having the Xe, W and Xe+W summation spectra. Energy intensity is normalized to wavelength in nanometers (nm). FIG. 1 shows the measured spectral intensity of the prior art xenon and tungsten dual source solar simulator as the Xe+W summation spectra and is compared to the AM0 spectra. The simulator spectra Xe+W summation has too much energy in the visible portion (500-700 nm), but matches AM0 very well in the IR. In addition, there are disadvantageously significant high intensity spikes beyond 600 nm wavelengths.
This prior art dual source simulator uses absorbing color glass filters to respectively isolate the xenon spectral component and the tungsten spectral component. The light from the tungsten source is filtered through a red lens which passes only visible and IR light. The light from the xenon source is filtered through a blue lens which passes only the UV and visible light. The simulator allows for easy measurement of the separate blue and red portions of the spectrum by simply blocking the respective lens with an opaque cover. FIG. 1 shows the separately measured xenon (Xe) blue spectral component and the tungsten (W) red spectral component. It is evident that the excess energy in the visible portion of the simulator spectrum is caused by the summation of the two components between 600-700 nm wavelengths. Additionally, the turn on of the pass band absorption edge curve of the tungsten source at about 575 nm and the turn off of the pass band absorption curve of the xenon source at about 675 nm are not ideally matched providing a high Xe-W summation level therebetween. Also, the filtering of the xenon light is not complete and several of the high intensity spikes are apparent in the measured Xe+W summation spectra. The far IR is not shown in the measured spectrum due to the limitations of the silicon photodetector, but other measurements have shown that the dual source solar simulator has too much energy beyond the 1100 nm wavelength. This is an expected function of the tungsten source, a 3200K blackbody, and the red color glass filter. Therefore, very low band gap materials, such as an active Ge junction in a GaAs/Ge solar cell or a GaInP.sub.2 /GaAs/Ge triple junction cell, would produce more measured current when illuminated by the solar simulator than under solar illumination in space. Another problem with the prior art Xe-W dual source solar simulator is the over heating of the color glass filters which absorb light energy when filtering unwanted spectral components leading to over heating and premature failure of the color glass filters.
Integral energy is used to give a quantitative measure of matching of the solar simulator to the AM0 standard. The integral energy is measured in discrete energy bins of narrow bandwidth segments over the entire bandwidth range for comparison between the solar simulators and the AM0 standard. Integral energy data for AM0 has been compared to the X-25 solar simulator. The X-25 solar simulator is fairly accurate over a wide wavelength range when measured in large energy bins. The integral energy data for AM0 has also been compared to the measured dual source solar simulator of FIG. 1. The dual source solar simulator has significantly lower energy in the 400-450 nm wavelength range and significantly extra energy in the 600-700 nm wavelength range.
Presenting the spectral data by only integral energy conceals the presence of high intensity spikes. The fidelity of the solar simulator refers to the smoothness of the spectral curve over very small wavelength ranges. A low fidelity solar simulator may have many high intensity spikes in its spectral output even though the integral energy and general shape of the spectrum may closely match the AM0 spectra. Therefore, both spectral intensity plots and the integral energy values must be considered when accurately comparing solar simulators to the AM0 spectra. This is particularly evident when comparing the X-25 and the prior art dual source simulators to the AM0 spectra. Each simulator has advantages while neither is ideal. The X-25 matches the AM0 spectra when comparing integral energy bins but has high intensity spikes. The dual source simulator has removed most of the xenon high intensity spikes but has an unbalanced integral energy profile.
A terrestrial AM1.5 dual source solar simulator has also been used. This design also uses a xenon arc lamp but with a "cold" mirror that reflects the UV and visual "cold" wavelength segments of the spectrum, closest to visual blue colors, and uses an incandescent source with a hot mirror that reflects the IR or "hot" wavelength segments of the spectrum, closest to visual red colors, respectively for cold and hot spectral segment matching to the AM1.5 spectra. However, there are imperfections in the hot and cold mirrors. In particular, the intensity of the xenon source is not completely filtered beyond the turn off wavelength of about 700 nm due to slight reflectivity in the infrared. The high intensity spikes are substantially reflected off of the cold mirror, but portions of the spikes are transmitted through the cold mirror and are apparent in the combined spectrum. While there are no very high intensity spikes remaining in the combined spectrum, the remaining smaller spikes are still a problem for accurate solar cell performance measurements. This terrestrial dual source solar simulator is not a significant improvement over the X-25 or prior art Xe-W dual source solar simulators.
Hence, there exists a continuing need for an improved solar simulator having a simulated spectra which matches the AM0 solar spectra, which provides the same integral energy, which removes unwanted intensity spikes, and which does not suffer from color glass filter over heating nor the use of respective spike filters to remove specific intensity spikes. These and other disadvantages are solved or reduced using the present invention.